1. Field of the Invention
The present invention is an optical device, and more particularly is to form a multi-layer structure over the substrate to increase the light reflective efficiency of the optical device.
2. Description of the Prior Art
The conventional light emitting diode (LED) as shown in FIG. 1A and FIG. 1B comprises a substrate 100, a buffer layer 102, a multi-layer structure, a transparent conductive layer 110 and two electrodes 112, 114. The multi-layer structure includes an active layer 106 between the N-type semiconductor layer 104 and the P-type semiconductor layer 108. The active layer 106 is a single-layer structure or a multi-layer structure made by a reactive nitrogen semiconductor compound. The voltage inputted between the electrodes 112, 114 of the LED will generate electrons and/or holes injecting into the N-type semiconductor layer 104 and P-type semiconductor layer 108 and passing through the active layer 106. The electrons and the holes will rejoin in the active layer 106 to generate light. The light generated in the illuminant layer will transmit to anywhere and release at the exposed surfaces of the LED. In order to generate the light, the light should be rejected at the predetermined direction.
Generally, the illuminant efficiency of the LED is based some parameters calculated at the LED. One is the light collection efficiency, which is the ratio of the light transmitted from the LED and the light generated by the LED. Practically, because of the absorption of the different layers within the LED, the light transmitted from the LED is less than the light generated by the LED. In order to increase the light collection efficiency, a reflective layer is added within the multi-layer structure of the LED in prior art to guide the light to transmit in the desired direction.
In order to improve the crystallized quality of the GaN compound layer, the problem of the lattice between the sapphire and the active layer of the GaN compound layer is needed to be solved. Therefore, in prior art, as shown in FIG. 1A and FIG. 1B, a buffer layer 102 made by MN is formed between the substrate 100 and the N-type semiconductor layer 104. The crystal structure of the buffer layer 102 is non-single crystal such as microcrystal or polycrystal. The crystal structure of the buffer layer 102 is able to solve the problem of the crystal mismatching between the GaN compound layers. However, the prior art described above, the refractive index generated by the light in the active layer restrains the illuminant efficiency of the optical device.